JPH09329745A - Switching type variable power optical system - Google Patents

Abstract

PROBLEM TO BE SOLVED: To suppress chromatic aberration and to obtain high optical performance from the center part of a picture to its periphery part while simplifying the structure of a lens-barrel by fixing the position of a 1st lens group and moving only the position of a 2nd lens group along its optical axis direction. SOLUTION: The optical system is provided with a 1st lens group G1 having negative refracting power and a 2nd lens group G2 having positive refracting power arranged successively from the object side. When only the 2nd lens group G2 is moved along the optical axis direction without moving the 1st lens group G1 along the optical axis direction, the magnification of image formation is switched while holding an image surface at a fixed state. When the focal distance of the 1st lens group G1 is f1 and the focal distance of the 2nd lens group G2 is f2, a condition of 0.3<f2/|f1|<0.5 is satisfied. When the system exceeds the upper limit value of the condition, the divergent action of the 1st lens group G1 is strengthened and the whole length is extended, and when the system is reduced lower than the lower limit value, the divergent action of the 1st lens group G1 is weakened and out-of-axis beam is too separated from the optical axis.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a switchable variable power optical system, and more particularly to a switchable variable power optical system for a finite distance with a constant object-image distance.

[0002]

2. Description of the Related Art In recent years, so-called original reading optical systems have been used in facsimiles, image scanners and the like. In a facsimile or an image scanner, a line sensor in which light receiving elements having a photoelectric conversion function are linearly arranged is mainly used. Then, a method of reading the original linearly while moving the original along a direction perpendicular to the arrangement direction of the light receiving elements is mainly used.

Optical design requirements for these original reading optical systems are the following three matters. Little distortion. High optical performance must be ensured in all areas from the center of the screen to the periphery of the screen. A sufficient amount of peripheral light is secured up to the periphery of the screen.

The document reading optical system is classified into a fixed magnification optical system having a constant lateral magnification and a variable magnification optical system having a variable lateral magnification. In particular, when the variable magnification optical system is used, not only a manuscript having a predetermined size but also a manuscript having an arbitrary size can be read. Also, in the variable magnification optical system,
High resolution can be obtained by capturing a part of the original document. As described above, the variable-magnification optical system has high versatility and is becoming the mainstream as a document reading optical system. Generally, the variable magnification optical system is classified into a method in which the object-image distance changes with a change in the imaging magnification, and a method in which the object-image distance is constant without depending on the change in the imaging magnification. To be done.

In the system in which the object-image distance is variable, the imaging magnification is changed by changing the object-image distance using a fixed focus lens. On the other hand, in the method in which the object-image distance is fixed, the focal length of the lens system is changed by using a zoom lens, thereby changing the imaging magnification while keeping the object-image distance constant. A lens system disclosed in, for example, Japanese Patent Application Laid-Open No. 60-122917 is known as a system in which the distance between object images is variable. Further, as a system in which the object-image distance is fixed, lens systems disclosed in, for example, JP-A-57-73715, JP-A-61-129613, and JP-A-6-230280 are known.

[0006]

When a system in which the object-image distance is variable is used, it is possible to effectively use the space by using a folding mirror composed of two mirrors. However, the amount of movement of the mirror and the lens system required when changing the imaging magnification is large, which is not suitable for downsizing the entire optical system. On the other hand, the method in which the object-image distance is fixed is suitable for downsizing the entire optical system and is suitable for a variable magnification optical system.

The lens system disclosed in Japanese Patent Laid-Open No. 57-73715 has a negative and positive two-group structure. Further, a symmetrical type is used in which the front and rear refractive power arrangements are substantially equal to each other with respect to the second lens group having a positive refractive power with the diaphragm interposed therebetween. Therefore, the refractive power arrangement in the entire lens system becomes asymmetric, and it is difficult to correct negative distortion. Also, JP-A-61-1296
In the lens system disclosed in Japanese Patent No. 13, the total length of the lens is shortened by arranging the second lens group with positive and negative refractive power arrangements. However, the total lens length at the wide-angle end is large, which is not suitable for downsizing. further,
In the lens system disclosed in Japanese Patent Laid-Open No. 6-230280,
The overall lens length was large relative to the diagonal length of the screen, and the entire optical system was large.

By the way, the size of the original has a predetermined size, and the image forming magnification at the time of reading the original is uniquely determined by the size of the light receiving element such as the line sensor. Therefore, in the document reading optical system, there is no particular need to continuously change the imaging magnification. Generally, a zoom lens requires a variator for performing a zooming action and a compensator for keeping an image surface at a constant position. As a result, since the lens barrel structure drives two or more lens groups independently of each other, there is a limit to cost reduction.

With the recent progress in microfabrication technology, there has been proposed a line sensor which is composed of smaller photodetectors and is capable of high resolution. In addition, with the spread of color printers, red (hereinafter referred to as “R”), green (hereinafter referred to as “R”)
Line sensors capable of color-separating and reading the three colors of "G") and blue (hereinafter "B") are predominant. In this way, the optical design specifications required for the document reading optical system have been further increased.

Specifically, in order to obtain a high resolution, it is necessary to obtain a high contrast with respect to a higher spatial frequency, and therefore, it is necessary that the residual aberration amount be smaller. Further, in order to perform color separation into RGB, it is necessary that the residual aberration be small from the center of the screen to the periphery of the screen. However, in the conventional document reading optical system, the three RGB colors have not been sufficiently corrected for aberrations individually, and the optical performance required as the document reading optical system has not been sufficiently secured.

The present invention has been made in view of the above-mentioned problems, and while simplifying the lens barrel structure, it suppresses chromatic aberration as much as possible and has a high optical performance from the central portion of the screen to the peripheral portion of the screen. An object is to provide a switchable variable magnification optical system for reading a document.

[0012]

In order to solve the above-mentioned problems, in the present invention, a first lens group G1 having a negative refractive power and a second lens group having a positive refractive power are arranged in order from the object side. G2 and by moving only the second lens group G2 along the optical axis direction without moving the first lens group G1 along the optical axis direction, while keeping the image plane at a constant position. The imaging magnification is switched, the focal length of the first lens group G1 is set to f1, and the second lens group G is set.
Provided is a switchable variable power optical system which satisfies the condition of 0.3 <f2 / | f1 | <0.5, where f2 is the focal length of 2.

According to a preferred embodiment of the present invention, the second
The lens group G2 has, in order from the object side, a positive lens group G2P having a positive refractive power and a negative lens group G2N having a negative refractive power, without moving the first lens group G1 along the optical axis direction. By moving only the second lens group G2 along the optical axis direction, the imaging magnification is switched while keeping the object-image distance at a predetermined finite distance, and the back focus in the state of the smallest imaging magnification is Bfw. , Y is half the screen diagonal length, f2N is the focal length of the negative lens group G2N in the second lens group G2, and the first lens group G1 is
2. When the focal length of is f1, the condition of 0.25 <Bfw / 2Y <0.77 0.4 <f2N / f1 <1.2 is satisfied. Double optics.

[0014]

BEST MODE FOR CARRYING OUT THE INVENTION First, the refractive power arrangement and the function of each lens group of a switching type variable power optical system according to the present invention will be described. 2. Description of the Related Art Conventionally, as a zoom lens for a single-lens reflex camera, a zoom lens having a negative / positive two-group structure is known. In this type of zoom lens, the second lens group having a positive refractive power functions as a variator and the first lens group having a negative refractive power functions as a compensator. In particular, the use magnification of the second lens group is set so as to change with the same magnification, and the position of the first lens group in the optical axis direction at the wide-angle end and the position of the first lens group in the optical axis direction at the telephoto end are set. It is known that the lens total length becomes the shortest when and are matched.

That is, in the conventional zoom lens having the negative and positive two-group structure, when the positions of the first lens group in the optical axis direction are the same at the wide-angle end and the telephoto end, focusing only on the wide-angle end and the telephoto end, Only the positions of the two lens groups in the optical axis direction change. Therefore, in the present invention, by appropriately setting the refractive power of the first lens group G1 and the refractive power of the second lens group G2, the position of the first lens group G1 is fixed in the optical axis direction, and the second lens group G1 is fixed. By moving only the position of G2 in the optical axis direction, the focal length of the entire optical system and thus the imaging magnification are switched. Particularly, in the present invention, the imaging magnification is switched while maintaining the object-image distance at a predetermined finite distance.

In the present invention, if only the second lens group G2 is moved in the optical axis direction, the focal length can be switched, that is, the imaging magnification can be switched. Therefore, the driving mechanism can be simplified as compared with the conventional variable power optical system that drives two lens groups to switch the focal length. Further, in the present invention, high optical performance is ensured only in the state where the imaging magnification is smallest, that is, the minimum magnification state (corresponding to the wide-angle end) and the largest imaging magnification, that is, the maximum magnification state (corresponding to the telephoto end). Since the specifications can be satisfied, it is possible to improve performance and simplify the lens configuration.

A variable power optical system based on an idea similar to the present invention is disclosed in Japanese Patent Laid-Open No. 59-201013. However, in the variable power optical system disclosed in JP-A-59-201013, the positive lens is arranged closest to the image plane of the second lens group, and the refractive power arrangement of the entire optical system is asymmetric. As a result, a large amount of negative distortion was generated, which was unsuitable for reading a document faithfully.

In the present invention, in the minimum magnification state, the first
By arranging the lens group G1 and the second lens group G2 at a predetermined distance and arranging an aperture stop in the second lens group G2, the off-axis light flux passing through the first lens group G1 is separated from the optical axis. On the other hand, it is possible to correct the axial aberration and the off-axis aberration independently of each other. On the other hand, in the maximum magnification state, the distance between the first lens group G1 and the second lens group G2 is made narrower than in the minimum magnification state, so that the first lens group G1
The off-axis light flux passing through the optical axis is brought close to the optical axis to favorably correct the fluctuation of the off-axis aberration that tends to occur at the time of switching the magnification.

Further, in the present invention, the positive lens group G2P including an aperture stop and the negative lens group G2N arranged on the image side thereof.
By configuring the second lens group G2 with and the principal point position of the second lens group G2 is located closer to the object than the center of the optical system, the total lens length can be shortened. Further, by constructing the second lens group G2 as described above, the refractive power arrangement in the entire optical system is approximated to a negative-positive / negative-symmetrical type, and the negative distortion that tends to occur in the minimum magnification state is favorably improved. It is possible to correct and read the original faithfully.

Further, by shortening the back focus in the minimum magnification state to some extent, the off-axis light flux passing through the negative lens group G2N is separated from the optical axis, and the on-axis aberration and the off-axis aberration are corrected independently of each other. Is possible. Further, correction of off-axis aberration is performed by the first lens group G1 and the negative lens group G.
By sharing it with 2N, the off-axis aberration is corrected well, and high optical performance is obtained not only in the center of the screen but also in the peripheral part of the screen.

In the maximum magnification state, since the second lens group G2 is moved to the object side more than in the minimum magnification state, the back focus becomes longer, and the off-axis aberration passing through the negative lens group G2N approaches the optical axis. Therefore, it is possible to suppress the fluctuation of the off-axis aberration that occurs at the time of switching the magnification. However, if the back focus in the minimum magnification state is too small, the off-axis aberration that passes through the negative lens group G2N is too far from the optical axis, the lens diameter becomes large, or a large positive distortion occurs. Therefore, even if the aperture efficiency is increased, the pupil diameter at a high image height is considerably smaller than the pupil diameter in the paraxial region, and it becomes difficult to obtain a predetermined peripheral light amount. Therefore, it is desirable to set the back focus to an appropriate value in the minimum magnification state.

In the variable power optical system, the positive lens group G2P in the second lens group G2 is the only lens group having a positive refractive power and has a strong positive refractive power. Therefore, it is necessary to satisfactorily correct the negative spherical aberration generated by the positive lens group G2P alone, and in the present invention, the positive lens group G2P has a lens shape that is advantageous for increasing the aperture. When the off-axis light flux passing through the positive lens group G2P moves away from the optical axis, the positive lens group G2P
However, it becomes necessary to correct not only the axial aberration but also the off-axis aberration at the same time, and the number of lens components increases. Therefore, in the present invention, the positive lens group G2P is divided into a first partial lens group having a positive refractive power arranged on the object side and a second partial lens group having a positive refractive power arranged on the image side, and is divided into two parts. By providing an aperture stop between the lens groups, high performance is achieved with a small number of lenses.

Further, in the present invention, the second lens group G2 is composed of the positive lens group G2P and the negative lens group G2N, and the negative refractive power of the first lens group G1 is the negative lens group G of the second lens group G2.
By shifting to 2N, the negative refracting power of the first lens group G1 is weakened negatively. Therefore, the number of lenses forming the first lens group G1 can be reduced. In addition,
If the off-axis light flux passing through the first lens group G1 is too far away from the optical axis, the lens diameter becomes large. Therefore, by configuring the first lens group G1 with a negative and positive refractive power arrangement, the off-axis light flux I try not to get too far from.

Each conditional expression of the present invention will be described below. In the present invention, the following conditional expression (1) is satisfied. 0.3 <f2 / | f1 | <0.5 (1) where f1: focal length of the first lens group G1 f2: focal length of the second lens group G2

Conditional expression (1) defines an appropriate range for the ratio between the focal length of the first lens group G1 and the focal length of the second lens group G2. If the upper limit of conditional expression (1) is exceeded, the diverging action of the first lens group G1 will be intensified, leading to an increase in the overall length of the lens. Conversely, conditional expression (1)
When the value is below the lower limit of, the divergence action of the first lens group G1 is weakened, and the off-axis light flux passing through the first lens group G1 is too far from the optical axis. As a result, when trying to obtain a predetermined amount of peripheral light in the minimum magnification state, the lens diameter becomes large. As described above, when the conditional expression (1) is not satisfied, the lens system becomes large in any case, which is inconvenient.

Further, in the present invention, in order to bring the refracting power arrangement of the entire optical system closer to a symmetric type and favorably correct negative spherical aberration, the second lens group G2 includes, in order from the object side, a positive refracting power. Positive lens group G2P and negative lens group G2N of negative refractive power
It is preferable that the following conditional expressions (2) and (3) are satisfied. 0.25 <Bfw / 2Y <0.77 (2) 0.4 <f2N / f1 <1.2 (3)

Here, Bfw: back focus in the minimum magnification state 2Y: diagonal length of the screen f2N: focal length of the negative lens group G2N in the second lens group G2

Conditional expression (2) is a conditional expression for defining the back focus in the minimum magnification state, and is a conditional expression for downsizing the optical system while maintaining a predetermined peripheral light amount. If the upper limit of conditional expression (2) is exceeded, the use magnification of the negative lens group G2N becomes positively large, and the diverging action is strengthened. As a result, the total length of the lens is increased, which is not preferable. On the contrary, when the value goes below the lower limit of the conditional expression (2), as described above, the positive distortion aberration generated in the negative lens group G2N increases, so that it becomes difficult to obtain a predetermined peripheral light amount. .

Conditional expression (3) is a conditional expression for defining the ratio between the focal length of the first lens group G1 and the focal length of the negative lens group G2N in the second lens group G2, and is generated when the magnification is switched. Is a conditional expression for satisfactorily correcting the fluctuation of coma aberration. If the upper limit of conditional expression (3) is exceeded, the focal length of the first lens group G1 becomes negative, so that the off-axis light flux passing through the first lens group G1 approaches the optical axis, and the coma aberration with respect to the downward light flux is reduced. Changes greatly during zooming. On the other hand, if the lower limit of conditional expression (3) is not reached, the focal length of the negative lens group G2N in the second lens group G2 becomes negative, so that the off-axis light flux passing through the negative lens group G2N becomes an optical axis. And the coma aberration with respect to the upper light flux changes greatly during zooming.

As described above, when the conditional expression (3) is not satisfied, in any case, the fluctuation of the coma aberration at the time of switching the magnification cannot be suppressed, and it becomes difficult to obtain a predetermined optical performance. It is not preferable because it will end up. In order to further reduce the total lens length while maintaining high optical performance, it is desirable to set the upper limit of conditional expression (3) to 0.9. Also, to further reduce the lens diameter,
It is desirable to set the lower limit of conditional expression (3) to 0.55.

As described above, in the present invention, the positive lens group G2P in the second lens group G2 is the only positive lens group. Therefore, in order to satisfactorily correct negative spherical aberration and suppress the occurrence of off-axis aberrations, the positive lens group G2P is divided into a first partial lens group having positive refractive power and a second partial lens group having positive refractive power. It is desirable to place an aperture stop between the two partial lens groups.

In the present invention, the second lens group G2 is moved in the optical axis direction when switching the image forming magnification. However, if the movement amount of the second lens group G2 at the time of switching the magnification is too small, the stopping accuracy required for the second lens group G2 becomes too high. Therefore, unless the driving mechanism that moves the second lens group G2 in the optical axis direction has a very high stop accuracy, the original image is often read in a blurred state, and the cost of the entire apparatus is reduced. It becomes unsuitable. On the contrary, the second lens group G2 when switching the magnification
If the amount of movement is too large, the amount of work of the drive mechanism becomes large, the structure cannot be simplified, and it becomes insufficient in terms of cost reduction.

Therefore, in the present invention, it is desirable to satisfy the following conditional expression (4). 0.6 <(DW-DT) / f2 <0.9 (4) Here, DW: axial air gap between the first lens group G1 and the second lens group G2 in the minimum magnification state DT: in the maximum magnification state On-axis air gap between the first lens group G1 and the second lens group G2 Conditional expression (4) defines an appropriate range for the movement amount of the second lens group G2 when switching the magnification. As described above, in the present invention, by satisfying conditional expression (4), it is possible to reduce the cost of the entire apparatus that controls the drive of the optical system.

Further, in the present invention, it is desirable that the following conditional expression (5) is satisfied. 0.1 <D / f2 <0.6 (5) where: D: On-axis between the lens disposed adjacent to the object side of the aperture stop and the lens disposed adjacent to the image side of the aperture stop Air gap

Conditional expression (5) is a conditional expression that regulates the air gap between the two lenses that sandwich the aperture stop. As described above, the positive lens group G2P mainly corrects the axial aberration, but the first lens group G1 and the negative lens group G2N completely correct the positive field curvature over the entire zoom range. It is difficult to correct. Therefore, the positive lens group G2P also needs to contribute to the correction of off-axis aberrations in the entire optical system. If the upper limit of conditional expression (5) is exceeded, the off-axis aberrations generated in the first partial lens group and the second partial lens group become too large, making it difficult to construct the positive lens group G2P with a small number of lenses. turn into. On the other hand, conditional expression (5)
Below the lower limit of, it becomes difficult to correct the axial aberration and the off-axis aberration independently of each other, and it becomes impossible to achieve high performance.

Further, in the present invention, in order to suppress the occurrence of lateral chromatic aberration as much as possible, the negative meniscus lens is arranged on the most image side in the second lens group G2, and the negative meniscus lens has the following conditional expression (6). It is desirable to be satisfied. 45 <ν1 (6) Here, ν1: Abbe number of the negative meniscus lens disposed closest to the image side in the second lens group G2. When the lower limit value of the conditional expression (6) is not reached, the chromatic aberration of magnification particularly in the minimum magnification state. Is increased and color misregistration occurs.

In the present invention, it is desirable that the following conditional expression (7) is satisfied. 0.45 <f2P / | f2N | <0.8 (7) where, f2P: focal length of the positive lens group G2P in the second lens group G2 f2N: focal point of the negative lens group G2N in the second lens group G2 distance

Conditional expression (7) defines an appropriate range for the ratio between the focal length of the positive lens group G2P and the focal length of the negative lens group G2N in the second lens group G2. Conditional expression (7)
Below the lower limit of, the coma aberration with respect to the upper light flux increases in the minimum magnification state, and it becomes impossible to obtain a predetermined optical performance. On the other hand, when the value exceeds the upper limit of conditional expression (7), negative spherical aberration generated in the positive lens group G2P increases, and the optical performance of the entire screen deteriorates, which is not preferable.

In the present invention, by using a glass having a high anomalous dispersion or a glass having a very small dispersion for the positive lens group G2P in the second lens group G2, the secondary dispersion is suppressed as much as possible, and the RGB dispersion. Even if the three colors are received by different line sensors, it is possible to reduce the color shift.

Further, in the present invention, the first lens group G
By moving either one of the first lens group G2 and the second lens group G2 or moving the first lens group G1 and the second lens group G2 integrally, the document position is displaced (positional shift) in the optical axis direction. It is possible to correct (focus) the fluctuation of the image plane position that occurs at the time. Particularly, by moving the first lens group G1 and the second lens group G2 so that the distance between the first lens group G1 and the second lens group G2 changes, the displacement (positional deviation) of the document position in the optical axis direction.
It is also possible to correct the fluctuation of the image plane position while correcting the fluctuations of the various aberrations due to satisfactorily.

In particular, when focusing is performed by moving the first lens group G1, the second lens group G2 that moves when switching the magnification.
Since it is possible to separate the first lens group G1 that moves during focusing from the first lens group by the operation, the movement control of the lens group becomes easy. Further, when the second lens group G2 is moved for focusing, the amount of movement during focusing is smaller than the amount of movement during magnification switching. Therefore, two position detection systems having different detection accuracies are prepared, a detection system having a higher detection accuracy detects the movement amount of the second lens group G2 at the time of focusing, and a detection system having a lower detection accuracy provides a magnification. It is also possible to detect the amount of movement of the second lens group G2 at the time of switching.

Further, as disclosed in Japanese Patent Laid-Open No. 57-11333, by shifting either one of the first lens group and the second lens group in the direction perpendicular to the optical axis, the CCD It is also possible to shift the readable range in such photoelectric conversion devices. In addition,
First lens group G1 that does not move in the optical axis direction when switching magnification
If you shift, without complicating the lens barrel structure,
A shift of the readable range can be realized.

Further, by introducing an aspherical surface into any of the lens groups, it is possible to easily achieve high performance. In particular, by introducing an aspherical surface into the negative lens group G2N in the first lens group G1 or the second lens group G2 that is away from the aperture stop, off-axis aberrations can be corrected well. In addition, the second lens group G disposed near the aperture stop
By introducing an aspherical surface into the positive lens group G2P in 2,
Needless to say, it is possible to easily increase the diameter.

[0044]

Embodiments of the present invention will be described below with reference to the accompanying drawings. FIG. 1 is a diagram showing the distribution of refractive power of a switching type variable power optical system according to each example of the present invention. As shown in FIG. 1, the variable power optical system according to each example of the present invention includes, in order from the object side, a first lens group G1 having a negative refractive power.
And a second lens group G2 having a positive refractive power. The second lens group G2 includes, in order from the object side, a positive lens group G2P having a positive refractive power and a negative lens group G2N having a negative refractive power. Then, by moving only the second lens group G2 to the object side along the optical axis direction without moving the first lens group G1 in the optical axis direction, the minimum magnification state ( The imaging magnification is switched from W) to the maximum magnification state (T).

[First Embodiment] FIG. 2 is a view showing the lens arrangement of a switching type variable magnification optical system according to the first embodiment of the present invention. In the variable power optical system of FIG. 2, the first lens group G1
Is composed of, in order from the object side, a biconcave lens L11 and a positive meniscus lens L12 having a convex surface facing the object side. The second lens group G2 includes, in order from the object side,
A biconvex lens L21, a cemented positive lens L22 of a biconvex lens and a biconcave lens, a negative meniscus lens L23 having a convex surface facing the object side, a biconvex lens L24, a positive meniscus lens L25 having a concave surface facing the object side, and a concave surface facing the object side. It is composed of a negative meniscus lens L26 directed toward the lens.

As described above, the biconvex lens L21, the cemented positive lens L22, the negative meniscus lens L23 and the biconvex lens L24 form a positive lens group G2P, and the positive meniscus lens L25 and the negative meniscus lens L26 form a negative lens group G2N. ing. An aperture stop S is provided between the cemented positive lens L22 and the negative meniscus lens L23 in the positive lens group G2P. White plate glass is inserted between the object and the first lens group G1 and between the second lens group G2 and the image plane. These white plate glasses are fixed when the magnification is changed. In the first embodiment, the on-axis spacing between the white glass sheet on the object side and the object is
It is 2.00.

The following table (1) lists the values of specifications of the first embodiment of the present invention. In Table (1), β is the imaging magnification,
FN represents the F number, FNO represents the effective F number, H represents the object height, and Y0 represents the maximum image height. Further, the surface number is the order of the lens surface from the object side along the traveling direction of the light beam, and the refractive index is the e-line (λ = 546.1n).
m). In the lens specifications of Table (1), a surface having a radius of curvature of ∞ (infinity) represents a plane. Further, although the radius of curvature of the surface representing the aperture stop S is ∞, there is no lens surface on the surface representing the aperture stop S.

[0048]

[Table 1] β = −0.1790 to −0.3781 FN = 3.83 to 4.72 FNO = 4.46 to 6.78 H = −110.51 to −53.66 Y0 = 20.42 surface Number Curvature radius Surface spacing Refractive index Abbe number 1 ∞ 3.00 1.52428 58.80 (white glass) 2 ∞ (d2 = variable) 3 -267.8331 1.20 1.77651 49.45 4 31.1151 4.40 5 37.1492 3.40 1.67765 32.17 6 175.9009 (d6 = variable) 7 40.8315 2.50 1.84503 43.35 8 -91.0419 0.10 9 22.7155 4.20 1.49926 82.52 10 -45.9596 1.20 1.80945 33.89 11 34.2655 1.00 12 ∞ 5.35 (aperture stop S) 13 28.6194 1.20 1.75455 35.19 14 18.2542 13.50 15 223.7545 3.40 1.62287 60.14 16 -36.3454 15.00 17 -22.6389 3.00 1.58 18 -20.1673 2.50 19 -23.0902 1.20 1.48914 70.41 20 -165.6851 (d20 = variable) 21 ∞ 0.80 1.52428 58.80 (white plate glass) 22 ∞ 1.20 (variable interval when switching magnification) β -0.1890 -0.3781 d2 232.9812 232.9812 d6 28.8950 1.7980 d20 17.9788 45.0758 (Value corresponding to the condition) f1 = -84.9021 f2 = 38.2972 Bfw = 19.9787 f2N = -69.9842 f2P = 40.477 However, Bfw is calculated in the state where white plate glass (protective glass) is excluded (1) f2 / | f1 | = 0.451 (2) Bfw / 2Y = 0.489 (3) f2N / f1 = 0.824 (4) (DW-DT) / f2 = 0.708 (5) D / f2 = 0.162 (6) ν1 = 70.41 (7) f2P / | f2N | = 0.578

FIGS. 3 and 4 are graphs showing various aberrations of the first embodiment. FIG. 3 is a diagram of various aberrations in the minimum magnification state,
FIG. 4 is a diagram of various types of aberration in the maximum magnification state. In each aberration diagram, NA is the numerical aperture, Y is the image height, H is the object height for each image height, e is the e-line (λ = 546.1 nm),
“G” indicates the g-line (λ = 435.8 nm). In the aberration diagram showing astigmatism, a solid line indicates a sagittal image plane, and a broken line indicates a meridional image plane. Further, in the aberration diagram showing the spherical aberration, a broken line indicates a sine condition (sine condition). As is clear from each aberration diagram, in this example, various aberrations are satisfactorily corrected in each imaging magnification state, and excellent imaging performance is ensured.

[Second Embodiment] FIG. 5 is a view showing the lens arrangement of a switching type variable magnification optical system according to the second embodiment of the present invention. In the variable power optical system of FIG. 5, the first lens group G1
Is composed of, in order from the object side, a biconcave lens L11 and a positive meniscus lens L12 having a convex surface facing the object side. The second lens group G2 includes, in order from the object side,
It includes a biconvex lens L21, a cemented positive lens L22 that is a combination of a biconvex lens and a biconcave lens, a positive meniscus lens L23 having a concave surface facing the object side, and a negative meniscus lens L24 having a concave surface facing the object side.

As described above, the biconvex lens L21, the cemented positive lens L22 and the positive meniscus lens L23 form a positive lens group G2P, and the negative meniscus lens L24 forms a negative lens group G2N. An aperture stop S is provided between the cemented positive lens L22 and the positive meniscus lens L23 in the positive lens group G2P. White plate glass is inserted between the object and the first lens group G1 and between the second lens group G2 and the image plane. These white plate glasses are fixed when the magnification is changed. In the second embodiment, the axial distance between the white glass sheet on the object side and the object is 2.00.

Table 2 below summarizes data values of the second embodiment of the present invention. In Table (2), β is the imaging magnification,
FN represents the F number, FNO represents the effective F number, H represents the object height, and Y0 represents the maximum image height. Further, the surface number is the order of the lens surface from the object side along the traveling direction of the light beam, and the refractive index is the e-line (λ = 546.1n).
m). In the lens specifications of Table (2), a surface having a radius of curvature of ∞ (infinity) represents a plane. Further, although the radius of curvature of the surface representing the aperture stop S is ∞, there is no lens surface on the surface representing the aperture stop S.

[0053]

[Table 2] β = −0.1790 to −0.3780 FN = 3.77 to 4.73 FNO = 4.47 to 6.99 H = −109.54 to −53.58 Y0 = 20.42 surface Number Curvature radius Surface spacing Refractive index Abbe number 1 ∞ 3.00 1.52428 58.80 (white glass) 2 ∞ (d2 = variable) 3 -774.2855 1.20 1.80832 46.51 4 30.6267 5.00 5 37.3384 3.40 1.72311 29.50 6 130.2154 (d6 = variable) 7 54.7353 2.50 1.80832 46.51 8 -78.2626 0.10 9 20.7766 7.25 1.49926 82.52 10 -38.2768 2.00 1.81184 33.27 11 22.6357 4.00 12 ∞ 11.00 (aperture diaphragm S) 13 -416.0174 3.00 1.80832 46.51 14 -33.5784 15.50 15 -21.6570 1.50 1.62286 60.35 16 -56.6042 (d16 = variable) ) 17 ∞ 0.80 1.52428 58.80 (white plate glass) 18 ∞ 1.20 (variable interval in magnification switching) β -0.1890 -0.3780 d2 233.0212 232.9998 d6 28.9529 1.8000 d16 24.5754 51.7493 (conditional corresponding value) f1 = -84.7793 f2 = 38.4182 Bfw = 26.5754 f 2N = -57.2607 f2P = 37.9894 However, Bfw is calculated in the state where the white plate glass (protective glass) is excluded. (1) f2 / | f1 | = 0.453 (2) Bfw / 2Y = 0 .651 (3) f2N / f1 = 0.675 (4) (DW-DT) / f2 = 0.707 (5) D / f2 = 0.390 (6) ν1 = 60.35 (7) f2P / | f2N | = 0.663

6 and 7 are graphs showing various aberrations of the second embodiment. FIG. 6 is a diagram of various aberrations in the minimum magnification state,
FIG. 7 is a diagram of various aberrations in the maximum magnification state. In each aberration diagram, NA is the numerical aperture, Y is the image height, H is the object height for each image height, e is the e-line (λ = 546.1 nm),
“G” indicates the g-line (λ = 435.8 nm). In the aberration diagram showing astigmatism, a solid line indicates a sagittal image plane, and a broken line indicates a meridional image plane. Further, in the aberration diagram showing the spherical aberration, a broken line indicates a sine condition (sine condition). As is clear from each aberration diagram, in this example, various aberrations are satisfactorily corrected in each imaging magnification state, and excellent imaging performance is ensured.

[Third Embodiment] FIG. 8 is a view showing the lens arrangement of a switching type variable magnification optical system according to the third embodiment of the present invention. In the variable power optical system of FIG. 8, the first lens group G1
Is composed of, in order from the object side, a biconcave lens L11 and a positive meniscus lens L12 having a convex surface facing the object side. The second lens group G2 includes, in order from the object side,
It includes a biconvex lens L21, a biconvex lens L22, a cemented negative lens L23 of a positive meniscus lens having a concave surface facing the object side and a biconcave lens, a biconvex lens L24, and a negative meniscus lens L25 having a concave surface facing the object side.

In this way, the biconvex lens L21, the biconvex lens L22, the cemented negative lens L23, and the biconvex lens L24.
Constitute a positive lens group G2P, and a negative meniscus lens L25
Constitutes a negative lens group G2N. An aperture stop S is provided between the cemented negative lens L23 and the positive meniscus lens L24 in the positive lens group G2P. White plate glass is inserted between the object and the first lens group G1 and between the second lens group G2 and the image plane. These white plate glasses are fixed when the magnification is changed. In the third embodiment, the axial distance between the white glass on the object side and the object is 2.00.

Table (3) below lists values of specifications of the third embodiment of the present invention. In Table (3), β is the imaging magnification,
FN represents the F number, FNO represents the effective F number, H represents the object height, and Y0 represents the maximum image height. Further, the surface number is the order of the lens surface from the object side along the traveling direction of the light beam, and the refractive index is the e-line (λ = 546.1n).
m). In the lens specifications of Table (3), a surface having a radius of curvature of ∞ (infinity) represents a plane. Further, although the radius of curvature of the surface representing the aperture stop S is ∞, there is no lens surface on the surface representing the aperture stop S.

[0058]

Table 3 β = −0.1790 to −0.3807 FN = 3.78 to 4.74 FNO = 4.46 to 6.97 H = −109.84 to −53.23 Y0 = 20.42 surface Number Curvature radius Surface spacing Refractive index Abbe number 1 ∞ 3.00 1.52428 58.80 (white glass) 2 ∞ (d2 = variable) 3 -700.6386 1.20 1.77621 49.61 4 29.4690 4.66 5 35.3646 3.40 1.69416 31.16 6 123.5014 (d6 = variable) 7 51.2046 2.50 1.71615 53.93 8 -90.7016 0.10 9 24.5180 3.50 1.49926 82.52 10 -69.1590 1.29 11 -47.5651 2.56 1.74690 49.23 12 -23.3009 2.00 1.75457 35.04 13 31.9199 4.00 14 ∞ 9.63 (aperture diaphragm S) 15 -674.8982 3.00 1.80832 46.51 16 -34.0458 13.15 17 -18.5 17 1.50 1.65426 58.44 18 -42.8092 (d18 = variable) 19 ∞ 0.80 1.52428 58.80 (white plate glass) 20 ∞ 1.20 (variable interval when switching magnification) β -0.1890 -0.3807 d2 233.0003 233.0003 d6 29.3224 1.6290 d18 28.1861 55.8795 (condition corresponding value) f1 = -85.3635 f2 = 38.2 79 Bfw = 30.1861 f2N = −51.0933 f2P = 40.7695 However, Bfw is calculated in a state excluding the white plate glass (protective glass) (1) f2 / | f1 | = 0.454 (2 ) Bfw / 2Y = 0.739 (3) f2N / f1 = 0.599 (4) (DW-DT) / f2 = 0.724 (5) D / f2 = 0.357 (6) v1 = 58.44 (7) f2P / | f2N | = 0.700

9 and 10 are various aberration diagrams of the third embodiment. FIG. 9 is a diagram of various aberrations in the minimum magnification state, and FIG. 10 is a diagram of various aberrations in the maximum magnification state. In each aberration diagram, NA is the numerical aperture, Y is the image height, H is the object height for each image height, and e is the e-line (λ = 546.1n).
m) and g indicate the g-line (λ = 435.8 nm). In the aberration diagram showing astigmatism, a solid line indicates a sagittal image plane, and a broken line indicates a meridional image plane. Further, in the aberration diagram showing the spherical aberration, a broken line indicates a sine condition (sine condition).
As is clear from each aberration diagram, in this example, various aberrations are satisfactorily corrected in each imaging magnification state, and excellent imaging performance is ensured.

[Fourth Embodiment] FIG. 11 is a view showing the lens arrangement of a switching type variable magnification optical system according to the fourth embodiment of the present invention. In the variable power optical system of FIG. 11, the first lens group G
Reference numeral 1 denotes, in order from the object side, a negative meniscus lens L11 having a convex surface facing the object side and a positive meniscus lens L12 having a convex surface facing the object side. The second lens group G2 includes, in order from the object side, a biconvex lens L21, a cemented positive lens L22 formed of a biconvex lens and a biconcave lens, a positive meniscus lens L23 having a concave surface facing the object side, and a concave surface facing the object side. It comprises a negative meniscus lens L24.

Thus, the biconvex lens L21, the cemented positive lens L22, and the positive meniscus lens L23 form a positive lens group G2P, and the negative meniscus lens L24 forms a negative lens group G2N. An aperture stop S is provided between the cemented positive lens L22 and the positive meniscus lens L23 in the positive lens group G2P. White plate glass is inserted between the object and the first lens group G1 and between the second lens group G2 and the image plane. These white plate glasses are fixed when the magnification is changed. In the fourth embodiment, the axial distance between the object-side white sheet glass and the object is 2.00.

The following table (4) lists the values of specifications of the fourth embodiment of the present invention. In Table (4), β is the imaging magnification,
FN represents the F number, FNO represents the effective F number, H represents the object height, and Y0 represents the maximum image height. Further, the surface number is the order of the lens surface from the object side along the traveling direction of the light beam, and the refractive index is the e-line (λ = 546.1n).
m). In the lens specifications of Table (4), a surface having a radius of curvature of ∞ (infinity) represents a plane. Further, although the radius of curvature of the surface representing the aperture stop S is ∞, there is no lens surface on the surface representing the aperture stop S.

[0063]

Table 4 β = −0.1873 to −0.3780 FN = 3.80 to 4.78 FNO = 4.45 to 6.94 H = −109.84 to −53.55 Y0 = 20.42 surface Number Curvature radius Surface spacing Refractive index Abbe number 1 ∞ 3.00 1.52428 58.80 (white glass) 2 ∞ (d2 = variable) 3 217.3806 1.20 1.80086 45.37 4 28.4034 6.80 5 34.6976 3.40 1.76168 27.53 6 67.6395 (d6 = variable) 7 51.1099 2.80 1.77074 46.80 8 -75.1121 0.10 9 21.2438 6.80 1.49926 82.52 10 -38.6473 2.00 1.80945 33.89 11 24.6119 3.50 12 ∞ 12.57 (Aperture stop S) 13 -465.9257 2.75 1.79192 47.47 14 -31.6905 11.00 15 -19.5524 1.50 1.69980 55.48 16 -46.6113 (d16 = variable) 17 ∞ 0.80 1.52428 58.80 (White plate glass) 18 ∞ 1.20 (Variable interval when switching magnification) β -0.1873 -0.3780 d2 230.0000 230.0000 d6 30.2650 2.1000 d16 28.3129 56.4779 (Values corresponding to conditions) f1 = -83.4949 f2 = 39.3112 Bfw = 30.5829 f2N = -49.2533 f2P = 36.4142 However, Bfw is calculated in a state excluding the white plate glass (protective glass) (1) f2 / | f1 | = 0.471 (2) Bfw / 2Y = 0. 749 (3) f2N / f1 = 0.675 (4) (DW-DT) / f2 = 0.702 (5) D / f2 = 0.409 (6) ν1 = 55.48 (7) f2P / | f2N | = 0.739

12 and 13 are various aberration diagrams of the fourth embodiment. FIG. 12 is a diagram of various aberrations in the minimum magnification state, and FIG. 13 is a diagram of various aberrations in the maximum magnification state.
In each aberration diagram, NA is the numerical aperture, Y is the image height, H is the object height for each image height, and e is the e-line (λ = 546.1n).
m) and g indicate the g-line (λ = 435.8 nm). In the aberration diagram showing astigmatism, a solid line indicates a sagittal image plane, and a broken line indicates a meridional image plane. Further, in the aberration diagram showing the spherical aberration, a broken line indicates a sine condition (sine condition).
As is clear from each aberration diagram, in this example, various aberrations are satisfactorily corrected in each imaging magnification state, and excellent imaging performance is ensured.

[Fifth Embodiment] FIG. 14 is a view showing the lens arrangement of a switching type variable magnification optical system according to the fifth embodiment of the present invention. In the variable power optical system of FIG. 14, the first lens group G
Reference numeral 1 denotes, in order from the object side, a negative meniscus lens L11 having a convex surface facing the object side and a positive meniscus lens L12 having a convex surface facing the object side. The second lens group G2 includes, in order from the object side, a biconvex lens L21, a cemented positive lens L22 formed of a biconvex lens and a biconcave lens, a positive meniscus lens L23 having a concave surface facing the object side, and a concave surface facing the object side. It comprises a negative meniscus lens L24.

Thus, the biconvex lens L21, the cemented positive lens L22, and the positive meniscus lens L23 form a positive lens group G2P, and the negative meniscus lens L24 forms a negative lens group G2N. An aperture stop S is provided between the cemented positive lens L22 and the positive meniscus lens L23 in the positive lens group G2P. White plate glass is inserted between the object and the first lens group G1 and between the second lens group G2 and the image plane. These white plate glasses are fixed when the magnification is changed. In the fifth example, the axial distance between the white glass sheet on the object side and the object is 2.00.

Table 5 below summarizes the data values of the fifth embodiment of the present invention. In Table (5), β is the imaging magnification,
FN represents the F number, FNO represents the effective F number, H represents the object height, and Y0 represents the maximum image height. Further, the surface number is the order of the lens surface from the object side along the traveling direction of the light beam, and the refractive index is the e-line (λ = 546.1n).
m). In the lens specifications of Table (5), a surface having a radius of curvature of ∞ (infinity) represents a plane. Further, although the radius of curvature of the surface representing the aperture stop S is ∞, there is no lens surface on the surface representing the aperture stop S.

[0068]

Table 5 β = −0.1907 to −0.3975 FN = 3.86 to 4.92 FNO = 4.53 to 7.26 H = −108.84 to −50.88 Y0 = 20.42 surface Number Curvature radius Surface spacing Refractive index Abbe number 1 ∞ 3.00 1.52428 58.80 (white glass) 2 ∞ (d2 = variable) 3 227.0609 1.20 1.80086 45.37 4 27.2216 8.20 5 34.5954 3.40 1.85504 23.83 6 57.2422 (d6 = variable) 7 47.0699 3.00 1.80086 45.37 8 -64.4602 0.10 9 18.7517 4.10 1.49926 82.52 10 -37.3089 4.70 1.80945 33.89 11 20.4861 3.50 12 ∞ 8.60 (Aperture stop S) 13 -218.0257 3.50 1.77651 49.45 14 -28.2709 12.10 15 -18.0291 2.00 1.69980 55.48 16 -37.7500 (d16 = variable) 17 ∞ 0.80 1.52428 58.80 (White plate glass) 18 ∞ 1.20 (Variable interval when switching magnification) β -0.1890 -0.3780 d2 188.9505 188.9505 d6 27.0875 0.9950 d16 22.5619 36.6004 (Conformance value) f1 = -72.0117 f2 = 34.7322 Bfw = 24.5619 f2N -51.4651 f2P = 35.9757 However, Bfw is calculated with the white plate glass (protective glass) removed (1) f2 / | f1 | = 0.482 (2) Bfw / 2Y = 0.601 (3) f2N / f1 = 0.715 (4) (DW-DT) / f2 = 0.751 (5) D / f2 = 0.348 (6) ν1 = 55.48 (7) f2P / | f2N | = 0.699

FIGS. 15 and 16 are various aberration diagrams of the first embodiment. FIG. 15 is a diagram of various aberrations in the minimum magnification state, and FIG. 16 is a diagram of various aberrations in the maximum magnification state.
In each aberration diagram, NA is the numerical aperture, Y is the image height, H is the object height for each image height, and e is the e-line (λ = 546.1n).
m) and g indicate the g-line (λ = 435.8 nm). In the aberration diagram showing astigmatism, a solid line indicates a sagittal image plane, and a broken line indicates a meridional image plane. Further, in the aberration diagram showing the spherical aberration, a broken line indicates a sine condition (sine condition).
As is clear from each aberration diagram, in this example, various aberrations are satisfactorily corrected in each imaging magnification state, and excellent imaging performance is ensured.

[0070]

As described above, according to the present invention, while simplifying the lens barrel structure, the chromatic aberration is suppressed as much as possible, and a document reading having a high optical performance from the central portion of the screen to the peripheral portion of the screen is provided. A switchable variable magnification optical system can be realized.

[Brief description of drawings]

FIG. 1 is a diagram showing a refractive power distribution of a switching type variable power optical system according to each example of the present invention.

FIG. 2 is a diagram showing a lens configuration of a switching type variable magnification optical system according to Example 1 of the present invention.

FIG. 3 is a diagram of various types of aberration in the minimum magnification state of the first example.

FIG. 4 is a diagram of various types of aberration in the maximum magnification state of the first example.

FIG. 5 is a diagram showing a lens configuration of a switching variable magnification optical system according to a second example of the present invention.

FIG. 6 is a diagram of various types of aberration in the minimum magnification state of the second example.

FIG. 7 is a diagram of various types of aberration in the maximum magnification state of the second example.

FIG. 8 is a diagram showing a lens configuration of a switching type variable magnification optical system according to a third example of the present invention.

FIG. 9 is a diagram of various types of aberration in the minimum magnification state of the third example.

FIG. 10 is a diagram of various types of aberration in the maximum magnification state of the third example.

FIG. 11 is a diagram showing a lens configuration of a switching type variable magnification optical system according to a fourth example of the present invention.

FIG. 12 is a diagram of various types of aberration in the minimum magnification state of the fourth example.

FIG. 13 is a diagram of various types of aberration in the maximum magnification state of the fourth example.

FIG. 14 is a diagram showing a lens configuration of a switching variable magnification optical system according to a fifth example of the present invention.

FIG. 15 is a diagram of various types of aberration in the minimum magnification state of the fifth example.

FIG. 16 is a diagram of various types of aberration in the maximum magnification state of the fifth example.

[Explanation of symbols]

 G1 First lens group G2 Second lens group Li Each lens component S Aperture stop

Claims (8)

[Claims] 1. A first lens group G1 having a negative refractive power and a second lens group G having a positive refractive power in order from the object side.
2 and by moving only the second lens group G2 along the optical axis direction without moving the first lens group G1 along the optical axis direction, while keeping the image plane at a constant position. The imaging magnification is switched, the focal length of the first lens group G1 is set to f1, and the second lens group G1 is
A switchable variable power optical system which satisfies the condition of 0.3 <f2 / | f1 | <0.5, where f2 is the focal length of the lens group G2. 2. The second lens group G2 includes, in order from the object side, a positive lens group G2P having a positive refractive power and a negative lens group G2N having a negative refractive power, and the first lens group G1 has an optical axis. By moving only the second lens group G2 along the optical axis direction without moving along the direction, the imaging magnification is switched while keeping the object-image distance at a predetermined finite distance, The back focus in a small state is Bfw, the half of the screen diagonal length is Y, and the second
The focal length of the negative lens group G2N in the lens group G2 is f2N
When the focal length of the first lens group G1 is f1, the condition of 0.25 <Bfw / 2Y <0.77 0.4 <f2N / f1 <1.2 is satisfied. Item 2. A variable magnification variable optical system according to Item 1. 3. The positive lens group G2P in the second lens group G2 includes, in order from the object side, a first partial lens group including at least two positive lenses and having a positive refractive power, and at least one lens. A second partial lens group including a positive lens and having a positive refractive power, and an aperture stop is provided between the first partial lens group and the second partial lens group. The variable magnification optical system according to claim 1 or 2. 4. The axial air gap between the first lens group G1 and the second lens group G2 in the state of the smallest imaging magnification is DW, and the first lens group G1 in the state of the largest imaging magnification. When the axial air gap between the second lens group G2 and the second lens group G2 is DT and the focal length of the second lens group G2 is f2, the condition of 0.6 <(DW-DT) / f2 <0.9 is satisfied. The switching variable magnification optical system according to claim 3, wherein the condition is satisfied. 5. An axial air gap between a lens disposed adjacent to the object side of the aperture stop and a lens disposed adjacent to the image side of the aperture stop is D, and the second lens group G
The switching variable power optical system according to claim 3 or 4, wherein the condition of 0.1 <D / f2 <0.6 is satisfied, where f2 is the focal length of 2. 6. The first lens group G1 has, in order from the object side, a negative lens and a positive lens, and the second lens group G2 has a biconvex lens disposed closest to the object side and the biconvex lens. 6. A cemented lens of a biconvex lens and a biconcave lens arranged on the image side of, and a negative meniscus lens having a concave surface facing the object side and arranged closest to the image plane. Or the switching variable magnification optical system described in the item 1. 7. The Abbe number ν1 of the negative meniscus lens arranged closest to the image side in the second lens group G2.
Satisfies the condition of 45 <ν1. 7. The switching type variable power optical system according to claim 6, wherein 8. When the focal length of the positive lens group G2P in the second lens group G2 is f2P and the focal length of the negative lens group G2N in the second lens group G2 is f2N, 0.45 8. The switching variable power optical system according to claim 2, wherein the condition of <f2P / | f2N | <0.8 is satisfied.